Selenate Reduction to Elemental Selenium by Anaerobic Bacteria in Sediments and Culture: Biogeochemical Significance of a Novel, Sulfate-Independent Respiration

Interstitial water profiles of SeO₄²⁻, SeO₃²⁻, SO₄²⁻, and Cl⁻ in anoxic sediments indicated removal of the seleno-oxyanions by a near-surface process unrelated to sulfate reduction. In sediment slurry experiments, a complete reductive removal of SeO₄²⁻ occurred under anaerobic conditions, was more rapid with H₂ or acetate, and was inhibited by O₂, NO₃⁻, MnO₂, or autoclaving but not by SO₄²⁻ or FeOOH. Oxidation of acetate in sediments could be coupled to selenate but not to molybdate. Reduction of selenate to elemental selenium was determined to be the mechanism for loss from solution. Selenate reduction was inhibited by tungstate and chromate but not by molybdate. A small quantity of the elemental selenium precipitated into sediments from solution could be resolublized by oxidation with either nitrate or FeOOH, but not with MnO₂. A bacterium isolated from estuarine sediments demonstrated selenate-dependent growth on acetate, forming elemental selenium and carbon dioxide as respiratory end products. These results indicate that dissimilatory selenate reduction to elemental selenium is the major sink for selenium oxyanions in anoxic sediments. In addition, they suggest application as a treatment process for removing selenium oxyanions from wastewaters and also offer an explanation for the presence of selenite in oxic waters.     

In situ bacterial selenate reduction in the agricultural drainage systems of western Nevada

Dissimilatory in situ selenate reduction to elemental selenium in sediments from irrigated agricultural drainage regions of western Nevada was measured at ambient Se oxyanion concentrations. Selenate reduction was rapid, with turnover rate constants ranging from 0.04 to 1.8 h-1 at total Se concentrations in pore water of 13 to 455 nM. Estimates of removal rates of selenium oxyanions were 14.38, and 155 mumol m-2 day-1 for South Lead Lake, Massie Slough, and Hunter Drain, respectively.     

In situ bacterial selenate reduction in the agricultural drainage systems of western Nevada.

Dissimilatory in situ selenate reduction to elemental selenium in sediments from irrigated agricultural drainage regions of western Nevada was measured at ambient Se oxyanion concentrations. Selenate reduction was rapid, with turnover rate constants ranging from 0.04 to 1.8 h-1 at total Se concentrations in pore water of 13 to 455 nM. Estimates of removal rates of selenium oxyanions were 14.38, and 155 mumol m-2 day-1 for South Lead Lake, Massie Slough, and Hunter Drain, respectively.     

Fate of Selenate and Selenite Metabolized by Rhodobacter sphaeroides

Cultures of a purple nonsulfur bacterium, Rhodobacter sphaeroides, amended with ~1 or ~100 ppm selenate or selenite, were grown phototrophically to stationary phase. Analyses of culture headspace, separated cells, and filtered culture supernatant were carried out using gas chromatography, X-ray absorption spectroscopy, and inductively coupled plasma spectroscopy-mass spectrometry, respectively. While selenium-amended cultures showed much higher amounts of SeO₃²⁻ bioconversion than did analogous selenate experiments (94% uptake for SeO₃²⁻ as compared to 9.6% for SeO₄²⁻-amended cultures from 100-ppm solutions), the chemical forms of selenium in the microbial cells were not very different except at exposure to high concentrations of selenite. Volatilization accounted for only a very small portion of the accumulated selenium; most was present in organic forms and the red elemental form.     

Prokaryotic Community Structure Driven by Salinity and Ionic Concentrations in Plateau Lakes of the Tibetan Plateau

The prokaryotic community composition and diversity and the distribution patterns at various taxonomic levels across gradients of salinity and physiochemical properties in the surface waters of seven plateau lakes in the Qaidam Basin, Tibetan Plateau, were evaluated using Illumina MiSeq sequencing. These lakes included Lakes Keluke (salinity, <1 g/liter), Qing (salinity, 5.5 to 6.6 g/liter), Tuosu (salinity, 24 to 35 g/liter), Dasugan (salinity, 30 to 33 g/liter), Gahai (salinity, 92 to 96 g/liter), Xiaochaidan (salinity, 94 to 99 g/liter), and Gasikule (salinity, 317 to 344 g/liter). The communities were dominated by Bacteria in lakes with salinities of <100 g/liter and by Archaea in Lake Gasikule. The clades At12OctB3 and Salinibacter, previously reported only in hypersaline environments, were found in a hyposaline lake (salinity, 5.5 to 6.6 g/liter) at an abundance of ∼1.0%, indicating their ecological plasticity. Salinity and the concentrations of the chemical ions whose concentrations covary with salinity (Mg²⁺, K⁺, Cl⁻, Na⁺, SO₄²⁻, and Ca²⁺) were found to be the primary environmental factors that directly or indirectly determined the composition and diversity at the level of individual clades as well as entire prokaryotic communities. The distribution patterns of two phyla, five classes, five orders, five families, and three genera were well predicted by salinity. The variation of the prokaryotic community structure also significantly correlated with the dissolved oxygen concentration, pH, the total nitrogen concentration, and the PO₄³⁻ concentration. Such correlations varied depending on the taxonomic level, demonstrating the importance of comprehensive correlation analyses at various taxonomic levels in evaluating the effects of environmental variable factors on prokaryotic community structures. Our findings clarify the distribution patterns of the prokaryotic community composition in plateau lakes at the levels of individual clades as well as whole communities along gradients of salinity and ionic concentrations.     

Resolution of Distinct Membrane-Bound Enzymes from Enterobacter cloacae SLD1a-1 That Are Responsible for Selective Reduction of Nitrate and Selenate Oxyanions

Enterobacter cloacae SLD1a-1 is capable of reductive detoxification of selenate to elemental selenium under aerobic growth conditions. The initial reductive step is the two-electron reduction of selenate to selenite and is catalyzed by a molybdenum-dependent enzyme demonstrated previously to be located in the cytoplasmic membrane, with its active site facing the periplasmic compartment (C. A. Watts, H. Ridley, K. L. Condie, J. T. Leaver, D. J. Richardson, and C. S. Butler, FEMS Microbiol. Lett. 228:273-279, 2003). This study describes the purification of two distinct membrane-bound enzymes that reduce either nitrate or selenate oxyanions. The nitrate reductase is typical of the NAR-type family, with α and β subunits of 140 kDa and 58 kDa, respectively. It is expressed predominantly under anaerobic conditions in the presence of nitrate, and while it readily reduces chlorate, it displays no selenate reductase activity in vitro. The selenate reductase is expressed under aerobic conditions and expressed poorly during anaerobic growth on nitrate. The enzyme is a heterotrimeric (αβγ) complex with an apparent molecular mass of ~600 kDa. The individual subunit sizes are ~100 kDa (α), ~55 kDa (β), and ~36 kDa (γ), with a predicted overall subunit composition of α₃β₃γ₃. The selenate reductase contains molybdenum, heme, and nonheme iron as prosthetic constituents. Electronic absorption spectroscopy reveals the presence of a b-type cytochrome in the active complex. The apparent K_m for selenate was determined to be ~2 mM, with an observed V_max of 500 nmol SeO₄²⁻ min⁻¹ mg⁻¹ (k_cat, ~5.0 s⁻¹). The enzyme also displays activity towards chlorate and bromate but has no nitrate reductase activity. These studies report the first purification and characterization of a membrane-bound selenate reductase.     

Dissimilatory Arsenate and Sulfate Reduction in Sediments of Two Hypersaline, Arsenic-Rich Soda Lakes: Mono and Searles Lakes, California

A radioisotope method was devised to study bacterial respiratory reduction of arsenate in sediments. The following two arsenic-rich soda lakes in California were chosen for comparison on the basis of their different salinities: Mono Lake (~90 g/liter) and Searles Lake (~340 g/liter). Profiles of arsenate reduction and sulfate reduction were constructed for both lakes. Reduction of [⁷³As]arsenate occurred at all depth intervals in the cores from Mono Lake (rate constant [k] = 0.103 to 0.04 h⁻¹) and Searles Lake (k = 0.012 to 0.002 h⁻¹), and the highest activities occurred in the top sections of each core. In contrast, [³⁵S]sulfate reduction was measurable in Mono Lake (k = 7.6 ×10⁴ to 3.2 × 10⁻⁶ h⁻¹) but not in Searles Lake. Sediment DNA was extracted, PCR amplified, and separated by denaturing gradient gel electrophoresis (DGGE) to obtain phylogenetic markers (i.e., 16S rRNA genes) and a partial functional gene for dissimilatory arsenate reduction (arrA). The amplified arrA gene product showed a similar trend in both lakes; the signal was strongest in surface sediments and decreased to undetectable levels deeper in the sediments. More arrA gene signal was observed in Mono Lake and was detectable at a greater depth, despite the higher arsenate reduction activity observed in Searles Lake. A partial sequence (about 900 bp) was obtained for a clone (SLAS-3) that matched the dominant DGGE band found in deeper parts of the Searles Lake sample (below 3 cm), and this clone was found to be closely related to SLAS-1, a novel extremophilic arsenate respirer previously cultivated from Searles Lake.     

Reduction of Selenite to Red Elemental Selenium by Rhodopseudomonas palustris Strain N

The trace metal selenium is in demand for health supplements to human and animal nutrition. We studied the reduction of selenite (SeO₃ ⁻²) to red elemental selenium by Rhodopseudomonas palustris strain N. This strain was cultured in a medium containing SeO₃ ⁻² and the particles obtained from cultures were analyzed using transmission electron microscopy (TEM), energy dispersive microanalysis (EDX) and X ray diffraction analysis (XRD). Our results showed the strain N could reduce SeO₃ ⁻² to red elemental selenium. The diameters of particles were 80–200 nm. The bacteria exhibited significant tolerance to SeO₃ ⁻² up to 8.0 m mol/L concentration with an EC₅₀ value of 2.4 m mol/L. After 9 d of cultivation, the presence of SeO₃ ²⁻ up to 1.0 m mol/L resulted in 99.9% reduction of selenite, whereas 82.0% (p<0.05), 31.7% (p<0.05) and 2.4% (p<0.05) reduction of SeO₃ ⁻² was observed at 2.0, 4.0 and 8.0 m mol/L SeO₃ ²⁻ concentrations, respectively. This study indicated that red elemental selenium was synthesized by green technology using Rhodopseudomonas palustris strain N. This strain also indicated a high tolerance to SeO₃ ⁻². The finding of this work will contribute to the application of selenium to human health.     

Evidence for Microbial Fe(III) Reduction in Anoxic, Mining-Impacted Lake Sediments (Lake Coeur d'Alene, Idaho)

Mining-impacted sediments of Lake Coeur d'Alene, Idaho, contain more than 10% metals on a dry weight basis, approximately 80% of which is iron. Since iron (hydr)oxides adsorb toxic, ore-associated elements, such as arsenic, iron (hydr)oxide reduction may in part control the mobility and bioavailability of these elements. Geochemical and microbiological data were collected to examine the ecological role of dissimilatory Fe(III)-reducing bacteria in this habitat. The concentration of mild-acid-extractable Fe(II) increased with sediment depth up to 50 g kg⁻¹, suggesting that iron reduction has occurred recently. The maximum concentrations of dissolved Fe(II) in interstitial water (41 mg liter⁻¹) occurred 10 to 15 cm beneath the sediment-water interface, suggesting that sulfidogenesis may not be the predominant terminal electron-accepting process in this environment and that dissolved Fe(II) arises from biological reductive dissolution of iron (hydr)oxides. The concentration of sedimentary magnetite (Fe₃O₄), a common product of bacterial Fe(III) hydroxide reduction, was as much as 15.5 g kg⁻¹. Most-probable-number enrichment cultures revealed that the mean density of Fe(III)-reducing bacteria was 8.3 × 10⁵ cells g (dry weight) of sediment⁻¹. Two new strains of dissimilatory Fe(III)-reducing bacteria were isolated from surface sediments. Collectively, the results of this study support the hypothesis that dissimilatory reduction of iron has been and continues to be an important biogeochemical process in the environment examined.     

Regulation of sulfate uptake by excised barley roots in the presence of selenate

Active transport of SO₄²⁻ and SeO₄²⁻ has been evaluated during 60-hour contact of barley roots with nutrient solutions containing either ³⁵SO₄²⁻ or ⁷⁵SeO₄²⁻, or both ions, at 0.1 milli-equivalent per liter. In the SO₄²⁻ solution the time course of active transport follows a straight line; if SeO₄²⁻ is also present transport is strongly inhibited after 20 to 30 hours for both ions. The S-Se uptake ratio remains 1.4 during the 60 hours; S-Se ratio shifts from 3.0 to 3.3 in proteins and falls to 0.6 in free amino acids. S-Se discrimination is mainly operating at the level of amino acid incorporation into proteins. The presence of Se-amino acids blocks this incorporation and brings about an accumulation of free amino acids; at the same time carrier activity is inhibited. The addition of methionine or Se-methionine causes a 60 to 80% inhibition of the active transport.     

Perfusion Method for Assaying Microbial Activities in Sediments: Applicability to Studies of N2 Fixation by C2H2 Reduction

A perfusion method for assaying nitrogenase activity (acetylene reduction) in marine sediments was developed. The method was used to assay sediment cores from Spartina alterniflora (salt marsh), Zostera marina (sea grass), and Thalassia testudinum (sea grass) communities, and the results were compared with those of conventional sealed-flask assays. Rates of ethylene production increased progressively with time in the perfusion assays, reaching plateau values of 2 to 3 nmol · g of dry sediment⁻¹ · h⁻¹ by 10 to 20 h. Depletion of interstitial NH₄⁺ was implicated in this stimulation of nitrogenase activity. Initial acetylene reduction rates determined by the perfusion assay of cores from the Spartina community ranged from 0.15 to 0.60 nmol of C₂H₄ · g of dry sediment⁻¹ · h⁻¹. These rates were similar to those for sediments assayed in sealed flasks without seawater when determined over linear periods of C₂H₄ production. Initial values obtained by using the perfusion method were 0.66 nmol of C₂H₄ · g of dry sediment⁻¹ · h⁻¹ for sediments from Zostera communities and 0.70 nmol of C₂H₄ · g of dry sediment⁻¹ · h⁻¹ for sediments from Thalassia communities. In all cases, rates determined by simultaneous slurry assays were lower than those determined by the perfusion method.     

Impact of Bacterial NO3? Transport on Sediment Biogeochemistry

Experiments demonstrated that Beggiatoa could induce a H₂S-depleted suboxic zone of more than 10 mm in marine sediments and cause a divergence in sediment NO₃⁻ reduction from denitrification to dissimilatory NO₃⁻ reduction to ammonium. pH, O₂, and H₂S profiles indicated that the bacteria oxidized H₂S with NO₃⁻ and transported S⁰ to the sediment surface for aerobic oxidation.     

Reduction of Sulfur Compounds in the Sediments of a Eutrophic Lake Basin

Concentrations of various sulfur compounds (SO₄²⁻, H₂S, S⁰, acid-volatile sulfide, and total sulfur) were determined in the profundal sediments and overlying water column of a shallow eutrophic lake. Low concentrations of sulfate relative to those of acid-volatile sulfide and total sulfur and a decrease in total sulfur with sediment depth implied that the contribution of dissimilatory sulfur reduction to H₂S production was relatively minor. Addition of 1.0 mM Na₂³⁵SO₄ to upper sediments in laboratory experiments resulted in the production of H₂³⁵S with no apparent lag. Kinetic experiments with ³⁵S demonstrated an apparent K_m of 0.068 mmol of SO₄²⁻ reduced per liter of sediment per day, whereas tracer experiments with ³⁵S indicated an average turnover time of the sediment sulfate pool of 1.5 h. Total sulfate reduction in a sediment depth profile to 15 cm was 15.3 mmol of sulfate reduced per m² per day, which corresponds to a mineralization of 30% of the particulate organic matter entering the sediment. Reduction of ³⁵S⁰ occurred at a slower rate. These results demonstrated that high rates of sulfate reduction occur in these sediments despite low concentrations of oxidized inorganic compounds and that this reduction can be important in the anaerobic mineralization of organic carbon.     

Capacity for Denitrification and Reduction of Nitrate to Ammonia in a Coastal Marine Sediment

The capacity for dissimilatory reduction of NO₃⁻ to N₂ (N₂O) and NH₄⁺ was measured in ¹⁵NO₃⁻-amended marine sediment. Incubation with acetylene (7 × 10⁻³ atmospheres [normal]) caused accumulation of N₂O in the sediment. The rate of N₂O production equaled the rate of N₂ production in samples without acetylene. Complete inhibition of the reduction of N₂O to N₂ suggests that the “acetylene blockage technique” is applicable to assays for denitrification in marine sediments. The capacity for reduction of NO₃⁻ by denitrification decreased rapidly with depth in the sediment, whereas the capacity for reduction of NO₃⁻ to NH₄⁺ was significant also in deeper layers. The data suggested that the latter process may be equally as significant as denitrification in the turnover of NO₃⁻ in marine sediments.     

Nitrogen Fixation Associated with the New Zealand Mangrove (Avicennia marina (Forsk.) Vierh. var. resinifera (Forst. f.) Bakh.)

Nitrogenase activity in mangrove forests at two locations in the North Island, New Zealand, was measured by acetylene reduction and ¹⁵N₂ uptake. Nitrogenase activity (C₂H₂ reduction) in surface sediments 0 to 10 mm deep was highly correlated (r = 0.91, n = 17) with the dry weight of decomposing particulate organic matter in the sediment and was independent of light. The activity was not correlated with the dry weight of roots in the top 10 mm of sediment (r = −0.01, n = 13). Seasonal and sample variation in acetylene reduction rates ranged from 0.4 to 50.0 μmol of C₂H₄ m⁻² h⁻¹ under air, and acetylene reduction was depressed in anaerobic atmospheres. Nitrogen fixation rates of decomposing leaves from the surface measured by ¹⁵N₂ uptake ranged from 5.1 to 7.8 nmol of N₂ g (dry weight)⁻¹ h⁻¹, and the mean molar ratio of acetylene reduced to nitrogen fixed was 4.5:1. Anaerobic conditions depressed the nitrogenase activity in decomposing leaves, which was independent of light. Nitrogenase activity was also found to be associated with pneumatophores. This activity was light dependent and was probably attributable to one or more species of Calothrix present as an epiphyte. Rates of activity were generally between 100 and 500 nmol of C₂H₄ pneumatophore⁻¹ h⁻¹ in summer, but values up to 1,500 nmol of C₂H₄ pneumatophore⁻¹ h⁻¹ were obtained.     

Beneficial Effects of Nickel on Pseudomonas saccharophila under Nitrogen-Limited Chemolithotrophic Conditions

Addition of nickel stimulated growth and nitrogenase activity of Pseudomonas saccharophila under nitrogen-limited chemolithotrophic conditions, apparently because of a significant increase in expression of uptake hydrogenase activity. Inhibition of hydrogenase expression by 50 μM EDTA was relieved by nickel over a wide concentration range (1 to 200 μM). Co²⁺, Zn²⁺, Mn²⁺, and Cu²⁺ stimulated expression of hydrogenase activity, but to a much lesser degree than nickel, and Fe²⁺, Mg²⁺, SeO₄²⁻, and SeO₃²⁻ did not increase expression. Nickel in individual combination with Mg²⁺, Fe²⁺, SeO₃²⁻, and SeO₄²⁻ resulted in activities that were essentially the same as that with nickel alone. Hydrogenase synthesis required the presence of nickel, and repression by O₂ was alleviated by increasing the concentration of added nickel. Cells placed under hydrogenase derepression conditions showed progressive incorporation of radioactive nickel to a much greater extent than did cells which were not derepressed.     

Early effects of salinity on nitrate assimilation in barley seedlings

The effect of NaCl and Na₂SO₄ salinity on NO₃⁻ assimilation in young barley (Hordeum vulgare L. var Numar) seedlings was studied. The induction of the NO₃⁻ transporter was affected very little; the major effect of the salts was on its activity. Both Cl⁻ and SO₄²⁻ salts severely inhibited uptake of NO₃⁻. When compared on the basis of osmolality of the uptake solutions, Cl⁻ salts were more inhibitory (15-30%) than SO₄²⁻ salts. At equal concentrations, SO₄²⁻ salts inhibited NO₃⁻ uptake 30 to 40% more than did Cl⁻ salts. The absolute concentrations of each ion seemed more important as inhibitors of NO₃⁻ uptake than did the osmolality of the uptake solutions. Both K⁺ and Na⁺ salts inhibited NO₃⁻ uptake similarly; hence, the process seemed more sensitive to anionic salinity than to cationic salinity.

Unlike NO₃⁻ uptake, NO₃⁻ reduction was not affected by salinity in short-term studies (12 hours). The rate of reduction of endogenous NO₃⁻ in leaves of seedlings grown on NaCl for 8 days decreased only 25%. Nitrate reductase activity in the salt-treated leaves also decreased 20% but its activity, determined either in vitro or by the `anaerobic' in vivo assay, was always greater than the actual in situ rate of NO₃⁻ reduction. When salts were added to the assay medium, the in vitro enzymic activity was severely inhibited; whereas the anaerobic in vivo nitrate reductase activity was affected only slightly. These results indicate that in situ nitrate reductase activity is protected from salt injury. The susceptibility to injury of the NO₃⁻ transporter, rather than that of the NO₃⁻ reduction system, may be a critical factor to plant survival during salt stress.

     

Effects of Carbon Substrates on Nitrite Accumulation in Freshwater Sediments

The contribution of the biochemical pathways nitrification, denitrification, and dissimilatory NO₃⁻ reduction to NH₄⁺ (DNRA) to the accumulation of NO₂⁻ in freshwaters is governed by the species compositions of the bacterial populations resident in the sediments, available carbon (C) and nitrogen (N) substrates, and environmental conditions. Recent studies of major rivers in Northern Ireland have shown that high NO₂⁻ concentrations found in summer, under warm, slow-flowing conditions, arise from anaerobic NO₃⁻ reduction. Locally, agricultural pollutants entering rivers are important C and N sources, providing ideal substrates for the aquatic bacteria involved in cycling of N. In this study a range of organic C compounds commonly found in agricultural pollutants were provided as energy sources in 48-h incubation experiments to investigate if the chemical compositions of the pollutants affected which NO₃⁻ reduction pathway was followed and influenced subsequent NO₂⁻ accumulation. Carbon stored within the sediments was sufficient to support DNRA and denitrifier populations, and the resulting NO₂⁻ peak (80 μg of N liter⁻¹ [approximate]) observed at 24 h was indicative of the simultaneous activities of both bacterial groups. The value of glycine as an energy source for denitrification or DNRA appeared to be limited, but glycine was an important source of additional N. Glucose was an efficient substrate for both the denitrification and DNRA pathways, with a NO₂⁻ peak of 160 μg of N liter⁻¹ noted at 24 h. Addition of formate and acetate stimulated continuous NO₂⁻ production throughout the 48-h period, caused by partial inhibition of the denitrification pathway. The formate treatment resulted in a high NO₂⁻ accumulation (1,300 μg of N liter⁻¹ [approximate]), and acetate treatment resulted in a low NO₂⁻ concentration (<100 μg of N liter⁻¹).     

Root absorption and transport behavior of technetium in soybean

The absorption characteristics and mechanisms of pertechnetate (TcO₄⁻) uptake by hydroponically grown soybean seedlings (Glycine max cv Williams) were determined. Absorption from 10 micromolar solutions was linear for at least 6 hours, with 30% of the absorbed TcO₄⁻ being transferred to the shoot. Evaluation of concentration-dependent absorption rates from solutions containing 0.02 to 10 micromolar TcO₄⁻ shows the presence of multiphasic absorption isotherms with calculated K_s values of 0.09, 8.9, and 54 micromolar for intact seedlings. The uptake of TcO₄⁻ was inhibited by a 4-fold concentration excess of sulfate, phosphate, selenate, molybdate, and permanganate; no reduction was noted with borate, nitrate, tungstate, perrhenate, iodate, or vanadate. Analyses of the kinetics of interaction between TcO₄⁻ and inhibiting anions show permanganate to be a noncompetitive inhibitor, while sulfate, phosphate, and selenate, and molybdate exhibit characteristics of competitive inhibitors of TcO₄⁻ transport suggesting involvement of a common transport process.     

Microbial Manganese and Sulfate Reduction in Black Sea Shelf Sediments

The microbial ecology of anaerobic carbon oxidation processes was investigated in Black Sea shelf sediments from mid-shelf with well-oxygenated bottom water to the oxic-anoxic chemocline at the shelf-break. At all stations, organic carbon (C_org) oxidation rates were rapidly attenuated with depth in anoxically incubated sediment. Dissimilatory Mn reduction was the most important terminal electron-accepting process in the active surface layer to a depth of ~1 cm, while SO₄²⁻ reduction accounted for the entire C_org oxidation below. Manganese reduction was supported by moderately high Mn oxide concentrations. A contribution from microbial Fe reduction could not be discerned, and the process was not stimulated by addition of ferrihydrite. Manganese reduction resulted in carbonate precipitation, which complicated the quantification of C_org oxidation rates. The relative contribution of Mn reduction to C_org oxidation in the anaerobic incubations was 25 to 73% at the stations with oxic bottom water. In situ, where Mn reduction must compete with oxygen respiration, the contribution of the process will vary in response to fluctuations in bottom water oxygen concentrations. Total bacterial numbers as well as the detection frequency of bacteria with fluorescent in situ hybridization scaled to the mineralization rates. Most-probable-number enumerations yielded up to 10⁵ cells of acetate-oxidizing Mn-reducing bacteria (MnRB) cm⁻³, while counts of Fe reducers were <10² cm⁻³. At two stations, organisms affiliated with Arcobacter were the only types identified from 16S rRNA clone libraries from the highest positive MPN dilutions for MnRB. At the third station, a clone type affiliated with Pelobacter was also observed. Our results delineate a niche for dissimilatory Mn-reducing bacteria in sediments with Mn oxide concentrations greater than ~10 μmol cm⁻³ and indicate that bacteria that are specialized in Mn reduction, rather than known Mn and Fe reducers, are important in this niche.